Since the development of the modern electrochemical cell in the eighteenth century, researchers have attempted to improve upon their performance, durability, and reliability. An electrochemical cell is generally defined as a device from which electricity is obtained as a result of a chemical reaction. A cell consists of two electrodes (an anode and a cathode) immersed in a solution (electrolyte). The chemical reaction take place between the two electrodes and the electrolyte. In a primary cell, current is produced directly as a result of a chemical reaction which is not reversible; however, in a secondary cell the chemical reaction is reversible and the cell can be charged by passing a current through it. Examples of electrochemical cells are dry cells, wet cells, standard cells, fuel cells, solid-electrolyte cells, and reserve cells. A battery is generally defined as a direct current and voltage source made up of one or more cells that convert chemical energy into electrical energy. A battery (and an electrochemical cell) generally consists of an anode, a cathode, a separator, and an electrolyte. Primary batteries, such as those found in a flashlight, for example, can not be recharged, unlike storage batteries, which can be recharged when a current in the reverse direction restores the original chemical state. The lead acid battery, used in automobiles, is the most common example of a storage battery. Electrolytes are generally defined as chemical compounds which when molten or dissolved in certain solvents, will conduct an electric current. In electrolytes the current is carried by positive and negative ions (cations and anions, respectively) rather than by electrons. These ions are present in fused ionic compounds, or in solutions of acids, bases, and salts, which dissociate into ions.
With respect to electrolytes, a class of molten compositions which is of particular interest in the field of electrochemical cells is the class of fused salt compositions which are molten at low temperature. It is important to note that not all fused salt compounds are necessarily molten; they will be solid or liquid based on the temperature. However, with respect to the present application, our discussion will be limited to those fused salt compounds which are also molten at low temperature. Such fused or molten salt compounds are mixtures of compounds (i.e. anions and cations) which are liquid at temperatures below the individual melting points of the component compounds. These mixtures commonly referred to as "melts", can form molten compositions simultaneously upon contacting the components together or after heating and subsequent cooling.
Some examples of low temperature molten or fused salts are chloroaluminate salts formed when alkylimidazolium or pyridinium salts are mixed with aluminum trichloride (AlCl.sub.3); and the chlorogallate salts formed by mixing gallium trichloride with methylethylimidazolium chloride. Aluminum trichloride, gallium trichloride, ferric chloride, and indium chloride belong to the class of compounds commonly referred to as metal halides. A metal halide is a compound consisting of a metal and a halogen, generally covalently bonded together. Other types of halides besides the chlorides are the fluorides, bromides, and iodides. Ambient temperature (generally between 20.degree. C. and 35.degree. C.) chloroaluminate melts for use as solvents in high energy rechargeable electrochemical cells have been under development for approximately fifteen years. These melts are generally made of aluminum trichloride and compounds such as N-(n-butyl)pyridinium chloride (BPC), 1-methyl-3-ethylimidazolium chloride (MEIC), trimethylphenylammonium chloride (TMPAC), trimethylsulfonium chloride (TMSC), and trimethylphosphonium chloride (TMPC).
Ambient temperature chloroaluminate melts may be used as electrolytes in the construction of electrochemical cells, batteries, photoelectrochemical cells, and capacitors. They may also be used in electrorefining and electroplating. For a melt to be used efficiently in such applications, it should preferably possess a wide electrochemical window, a high electrical conductivity, and be a liquid over a wide composition range. The electrochemical window is defined as the difference between the anodic and cathodic decomposition voltages of the melt, while the electrical conductivity is generally defined as the ratio of electric current density to the electric field in materials. Among the ambient temperature chloroaluminate melts mentioned above, the AlCl.sub.3 /BPC and the AlCl.sub.3 /MEIC melts exhibit the best physical and electrochemical properties. Further the AlCl.sub.3 /MEIC melt exhibit better properties than the AlCl.sub.3 /BPC melt. For example the AlCl.sub.3 /BPC melt has a relatively narrow electrochemical window, due to the reduction of the butylpyridinium cation, while melts containing MEIC have a wider electrochemical window. The electrical conductivities of the ambient temperature melts range from approximately 2 to 17 mS/cm, at 25.degree. C.
When these molten salts are used in applications that involve strong oxidizing agents, such as in high voltage batteries, the organic cation should preferably be stable towards strong oxidation. The stability of the organic cation towards oxidation can be determined by observing the reactivity of SCl.sub.3.sup.+ ion with the melt, by Raman spectroscopy. The SCl.sub.3.sup.+ ion is a very strong oxidizing agent. Therefore, cells constructed with cathodes containing the SCl.sub.3.sup.+ ion have produced rechargeable cells with a voltage &gt;4.2 V, which is among the highest voltages known for rechargeable cells. Studies of the stability of the SCl.sub.3.sup.+ ion in the presence of the 1-methyl-3-ethylimidazolium cation (MEI.sup.+) showed that the SCl.sub.3.sup.+ ion decomposed rapidly at ambient temperature (25.degree. C.). In the presence of the butylpyridinium cation (BP.sup.+) the SCl.sub.3.sup.+ ion appeared to be more stable. When tested at 60.degree. C., the concentration of the SCl.sub.3.sup.+ ion decreased approximately 87% in 6 days (i.e. approximately 15% per day). Therefore, there is a need for molten salt compositions which will be able to tolerate the presence of strong oxidizing agents over a wide temperature gradient.